(1) Field of the Invention
This invention relates to prefoamed propylene polymer-base particles, expansion-molded propylene polymer-base articles produced from the particles, and a process for the production of the expansion-molded polypropylene-base articles.
(2) Description of the Prior Art
The present applicant has, for many years, proceeded with researches on the production of expansion-molded articles by molding prefoamed propylene polymer-base particles, namely, by the so-called steam chest molding process. In the steam chest molding process, prefoamed particles are usually subjected to molding after an inorganic gas or a gaseous mixture of an inorganic gas and a volatile foaming agent has been introduced into the prefoamed particles and an internal pressure has thus been built up within the prefoamed particles. Even when prefoamed particles which had been pressurized and aged under the same conditions were molded successively into expansion-molded articles, the thus-obtained articles were in many instances varied in appearance and various properties such as flexibility, shrinkage factor, compression hardness, compression set and interparticle fusion-bonding property. On some occasions, their quality was poor. This tendency was particularly remarkable with highly-prefoamed particles.